


Moral Decay

by Mercy_Run



Category: Undertale (Video Game)
Genre: Angst, BloodyKetchup, Denial, Dirty Talk, Ecto-Penis (Undertale), Ecto-Tongue (Undertale), Ecto-Vagina (Undertale), Heat Fic, Horrortale Papyrus, I have a size kink and headcanon Horrortale monsters as huge, Language, M/M, Mildly Dubious Consent, Mindbreak, PTSD, Panic Attack, Post-Undertale Genocide Route, Sans is unstable, Smut, Switching Genitalia, Undertale Sans, Yep that ship name works, fite me, messy sex
Language: English
Status: Completed
Published: 2017-06-14
Updated: 2017-06-14
Packaged: 2018-11-14 04:19:24
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 2,935
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/11200296
Author URL: https://archiveofourown.org/users/Mercy_Run/pseuds/Mercy_Run
Summary: He had been alone…  Nothing but dust and silence to keep him company as Sans waits for another reset.  What he got instead was a terrifying version of Papyrus.  At first, Sans had been afraid but then he couldn’t help but take comfort in the unusual turn of company.It was all going to reset anyway.





	Moral Decay

**Author's Note:**

> Made this a separate work as it isn't like my usual themes, much like Riveted. Got this as a request and it sounded like a challenge. Horrortale Papyrus and Undertale Sans, which I am calling Bloodyketchup after it was suggested.
> 
> I guess read and see if you like it? X'D
> 
> NSFW Blog: http://mercy-run.tumblr.com

It shouldn’t have happened this way.

Why did the human get to decide who lived and who died?

Papyrus…

The lady at the door. Snowdin’s Residents. The Royal Guard. Undyne. Alphys. Mettaton. The denizens of Waterfall and Hotland. The Capital… The King.

Even the wilted and scattered petals just before the barrier…

There had been some survivors from the evacuation. Grillby had been one of them; the monster where he had teleported to so he wouldn't die alone. But instead of him dying… His mind flickered with the image of his old friend offering a sad smile as he used up everything he had; so that the skeleton might live. Sans hadn’t moved from the bar for a long time; staring at the dust scattered around him, on him with empty sockets as sobs wracked his body.

Grillby hadn’t deserved to die, especially for someone that had failed to stop the creature that had left the Underground empty.

And it was completely empty. Silent save for the ambient noises of weather and wind carrying what seemed like a never ending circulation of Dust.

Devoid of life.

Except Sans was still here.

At first, he hadn’t appreciated the irony. But now, whenever he bothered to move, dim eyes searching his surroundings… He couldn’t help but break into uncontrolled laughter. This was the best joke ever! How funny! This was his punishment for failing over and over, so why shouldn’t Sans appreciate the cosmic joke he had become?! But every time the small monster had given into such a wild fit of unhinged glee; he couldn’t stop the tears that followed…

Why?

This was what he deserved. To die here, like every other member of his species.

The question of why the skeleton was still persisting and hadn’t Fallen was something on the edge of his mind constantly. Why couldn’t he just die? Sans had 1 HP, he shouldn’t have lasted this long. But he didn’t die and he was too lazy (cowardly) to take his own life. So instead of Falling, Sans had taken a seat at his sentry station; Papyrus’ scarf wrapped around his neck for warmth that never came.

He had been here for what felt like forever. But then…

Something moved in the tree-line, causing faint eye-lights to flicker to life in his sockets to shift absently towards it. The skeleton remained absolutely still, confused and disoriented. No one else was here. Sans had checked over and over. No one. Why movement? Everything was still and after waiting for a long moment, he was about to dismiss it as a trick of his lonely soul only-

-to spot two vivid crimson lights glowing in the darkness of Snowdin’s Forest. Watching him.

His small body tensed in the wooden seat, eye-lights brightening and shrinking on that point in the forest. No. No... He was seeing things. Sans had finally gone insane. There was no one down here! He had checked… He had checked! But he couldn’t help himself... Sans called out, his normally smooth and deep voice now shaky and weak from disuse.

“h-hello?”

The twin points of light vanished, as if never having been there at all. Sans stared for a long minute before laughing shakily. Just a trick on his mind, nothing m-

_Crunch._

Sans’ pupils vanished.

_Crunch. Crunch._

Something was coming out of the woods; the sound of heavy footfalls driving the small skeleton to jump to his feet, swaying unsteadily as his soul pounded with terror. Sans wasn’t sure he wanted anyone here with him now... No matter how lonely, no matter how quiet. Because he knew-

There had only been him down here!

_Crunch. Crunch._

A figure emerged from the woods with a shambling shuffle, displacing snow as it traveled. It was massive but clearly bent over as if it couldn’t support it’s weight properly. Sans only had Asgore as a fitting comparison and his bones trembled, making light noises as the monster froze in place.

How had it gotten here? What was it? What did it want? It smelled like decay, pine and-

Tomato sauce.

Sans stood there, unable to move a single phalange as the creature closed the distance.

**Crunch. Crunch. Crunch.**

No…

No. This couldn’t be! His empty gaze took in the angular and deformed arches of bones, the dirty clothing, a face that was too wrong but also too familiar and… A hand trembled as it reached up and grasped the red scarf around his neck for comfort, a habit now. There was something similar around the creature’s neck.

Sans had to be having a nightmare or his mind had finally broken. It was overdue anyway.

But the creature still stood there, closing the last bit of polite distance with heavy steps.

**Crunch.**

A whiff of a metallic scent had his tiny eye-lights moving to the jagged teeth to see what had to be-

Sans’ soul seized and the skeleton cried out sharply as his other hand flew to his chest, eye sockets vacant as his mind filled with blood, laughter, a child’s taunting gaze and pain-oh stars, it hurt! Phalanges trembling against the scar time wouldn’t heal; Sans realized he couldn’t seem to capture his breath. Rasping out frosty puffs of air quicker and quicker, darkness eating at the edges of his vision, he didn’t realize-

Crunch.

-it was too close! So much that Sans would practically break his neck in order to look at it. The barest of whines escaped him and squeezing his eye-sockets shut, the monster waited for it to finally kill him.

“…thr…?”

It was a voice, barely heard above his frantic gasps for air. Cyan magic built up in his sockets as he stared at the creature’s legs, spilling down his face. A large hand reached towards him and-

Gently brushed them away. Unable to comprehend this, Sans wheezed and looked up until his neck hurt, the thing taking a step back so that their eyes could meet. That crimson magic was back in it’s sockets as the small skeleton’s face was caressed with the utmost gentleness.

“B-rother…”

It was thin and reedy, like the sound of wind through leaves but despite that- Sans recognized that voice and he couldn’t help the sobs that wracked his body as his mind finally wrapped around what it had desperately been trying to ignore. What was there but so twisted that it shouldn’t be possible. Trembling, he reached up with both hands and the towering skeleton monster dipped down, almost double so that Sans could cup his face, stroking his features as fresh tears spilled.

“p-paps-!”

Oh stars, this was his brother! Sans **KNEW** it was his brother! This was Papyrus! His soul knew the other as well as his own.

How had he failed Papyrus so horribly…?

Sans blacked out.

_____________________________________________________

Over the next few days; Sans grew heavily dependent on his new brother’s company. He would panic anytime Papyrus was out of sight. Logically, he knew this wasn’t his time-line’s brother, but the skeleton couldn’t help it. The other skeleton was strange too, so different that he shouldn’t confuse them. But, at the end of the day, it didn’t matter.

This was Papyrus.

Despite how he had been confused at having ample food in the refrigerator, something that Papyrus had frantically consumed as if starved to death. Despite having a terrifying presence and appearance. This was still his brother.

So he took care of his younger sibling, washing the…remnants of filth away from dull bones until they were mostly white again. Returning the more intact red scarf and remaking the battle body Sans knew he loved.

Papyrus was so quiet too. Something that was unsettling. He also watched Sans as if he was the most fascinating thing ever.

It should bother him.

But it didn’t.

He wasn’t alone. Papyrus was back. Sans didn’t want to Fall anymore.

_____________________________________________________________

One morning, when Sans opened his eyes; he was immediately met with pulsing scarlet eye-lights not an inch away from his own. Yelping in surprise, the small skeleton jerked in his bed and then nervously tried to laugh it off. Why was his brother in his room? Much less caging him against the mattress entirely so that his only way out was teleportation? Not that he would hurt Papyrus’ feelings like that. He had acted frightened once around his sibling and the crestfallen expression that had met him wasn’t worth it.

“p-papyrus? what are you doing in my room?” He questioned, voice still weak and a ghost of what it had been. His younger brother stared down at him, skull flushed with red magic.

“…H-hot…” Papyrus complained and Sans was instantly worried, reaching up to lay a hand on the other’s forehead. Brow furrowing in concern, he realized his sibling was nearly burning to the touch.

Was he sick? If he remembered right, Alphys lab was still stocked with medicine to help with these things. If nothing else, then the evacuation center would have what was needed. A reedy whimper was issued past those sharp fangs as Sans retracted his hand.

“i’m sorry, bro. i’ll go get you some medicine. just let me up and i will be-”

“N-No!” Papyrus insisted and Sans found himself pulled tightly into the other’s long arms. Stars, Papyrus was hot to the touch everywhe- His pupils vanished as something hard and hot ground against his tail-bone. Oh… A quick glance up at the other monster revealed the other monster was panting for air, a long serpentine red tongue lolling from his jaw.

“paps…you’re..?”

“H-Help me, brother,” his disfigured sibling pleaded, causing the small skeleton to shudder when that searing ecto-tongue licked a long stripe down the side of his skull to his neck, leaving a hot trail.

He couldn’t!

…could he?

No, this wasn’t right? Wouldn’t this be taking advantage? Sans would never do that to Papyrus.

“paps-i—ah!”

The red cock straining against the small shorts Sans had recreated ground up abruptly against the diminutive skeleton’s sacrum, clouding his already confused thoughts. That felt so fucking good-! No. He had to tell Papyrus what was going on and how to deal with it.

“papyrus, you’re in heat. you can-”

“Help me.” The soft plea was needy and broken even as that mouth closed around his fragile neck, scraping his vertebrae. Sans gasped and had to try with all his might to hold back the magic that wanted to answer, already pooling to his pelvis as Papyrus’ clawed hand ripped into his clothing. He had to fight this…had to-

“paps…?” Sans couldn’t think as his small body was pressed into his poorly kept mattress, hazy eye-lights staring up as his sibling who dwarfed him in a strange, thrilling way. Papyrus continued to shred his clothes with one hand and immediately caress the exposed bone hungrily. The other’s scent was sweet and suffocating, a good five inches of weeping cock poking out of the small jeans, the rest barely held in by the fabric. Sans shuddered, uncertain and vulnerable for the first time he remembered around his baby brother. That look Papyrus wore was predatory now and confident.

“Play with me, Sans.” Papyrus whispered enticingly.

Where had his younger brother learned to talk like this…?

His shirt was now barely covering anything, shredded to ribbons by careful claws. The gigantic skeleton smiled down at Sans, thumbing the blue budge that had formed in his shorts.

“Let’s play..?” The skeleton murmured, leaning down until that sinful tongue slipped past his teeth; pressing in just the right places inside that had Sans conjuring his own. Their shared kiss was disordered, careless and intense. The monster had been so lonely and hungry for touch without Papyrus constantly carrying him, hugging him, holding his hand…

“hah…” Their combined magic dribbled down his chin and hit his collar bone, “s-stars…paps…!”

His baby brother scrapped sharp fangs against neck and Sans was suddenly keeping the other there, clinging to Papyrus’ skull.

“please!”

Sans cock was freed faster than he had ever experienced before, his shorts meeting the same fate as his shirt. His hazy eye-lights darted up to check Papyrus’ face, only seeing hunger and lust there. The larger hand curled around his shaft and began practiced movements that the smaller skeleton was soon canting his hips up into eagerly. His brother was confident and for the first time ever, he felt overpowered and nearly helpless. In a way, he felt like he was. Never had he been able to deny his sibling anything. And Papyrus was something truly monstrous, like the human fables. Sans would easily fit into his ribcage or carried on a single shoulder.

Sans wanted to give himself away.

Have his baby brother fuck him and make him scream.

No one was here to hear them.

No one to tell them this was wrong.

Only them.

He curled one hand around a floating rib, tugging until Papyrus was kissing him, magic slipping past no matter how quickly Sans swallowed. The slick of his pre made sinful noises as his sibling jerked him off. He thrust up in time with the grip, creating a chaotic pace as Papyrus’ hand moved from his hip only for one clawed phalange to circle the entrance just below. Sans shuddered and whined through his teeth, baring his rarely seen fangs as it slipped carefully in just barely and out. A soft rocking touch that had his hole loosening as he relaxed, sliding in and filling him, his dick jerked perfectly in time-

“fuck! ah!”

His member twitched repeatedly, spurting cyan magic from the sensitive tip as Sans came harder than he ever had before in his life. A low rumbling drew the skeleton out of his blissed out daze to see the huge red ecto-cock being freed, his brother growling in hunger.

“Change your magic, Sans.”

Understanding touched the smaller monster’s eye-lights and he focused, his magic fading into a light blue slit that still had wetness trickling out. Wanting more, he turned over and got on his hands and knees. This seemed to please Papyrus, the reedy snarl now a deep purr as his small body was pulled backwards, nearly completely engulfed and hidden as the larger skeleton lined them up.

His soul trembled as the heated tip rubbed around his small lips before it became immediately apparent that Papyrus would tear him apart-

“w-wait!” He cried, “you’ll b-break me-!”

The purr deepened as Papyrus laughed lowly and a large hand reached between his femurs to rub circles on the bud of magic at the top of his cunt, drawing a full body shudder.

“I believe y-you can help me, brother. Spread yourself for me.”

“ahnn!”

Fuck morals, Sans had never wanted anything more than how he needed Papyrus inside him right now. He obeyed and that touch continued to circle his clit; slick oozing from where they were barely connected. A line of blue leaking onto the scarlet shaft as the smaller monster moaned, grinding against the stimulation. Sans was so lost in the sensation that he nearly missed the insistent press forward of his sibling’s pelvis.

Papyrus slid in a few inches, pulsing, rigid and so hot that Sans keened in response, finding himself trapped completely. His wrists were suddenly captured and pulled back until gravity was doing most of the work, his spine pulled into an arch. The only part of him still touching the mattress was his heels, grinding into the mattress as he screamed; his translucent walls stretched and quivering as Papyrus’s cock caught at the widest part.

With one decisive thrust, Sans felt his mind white out as places he didn’t even know he had were filled completely.

“S-So good, Sans. So hot inside!”

And then Papyrus was moving, the slow drag of ecto-flesh until there was nothing but the tip inside only to be quickly filled back up to near bursting. There was no finesse to how his baby brother fucked him, it was wild and a thing of pleasure-seeking.

And Sans was loving every moment of it.

Two fingers that tasted like his magic were shoved past his teeth and held his mouth open, his keens, whimpers and moans spilling freely. He was drooling all over them but he couldn’t care. He never wanted this to stop, the squelch of their joined magic causing him to clench and whine in warning.

But the other skeleton didn’t seem to care to stop either, ignoring the scream past magic-messy fingers and fucking him even as his body tried it’s damnedest to force Papyrus’ cock out as it tightened. Right through his orgasm and turning a blindness to his overstimulated mewling, his magical flesh quivered as Sans had no choice but to take what he was given.

He came a second time and then a third before-

“B-Brother!”

Sans choked as his small pelvis was slammed flush with Papyrus’, his cunt throbbing as a rush of heated magic quickly filled him up, overflowing and leaking down his shaking femurs. Papyrus whimpered and whispered soft, reedy words of affection against his skull before being before eased back into his brother’s lap, still joined and dazed.

“You d-did so well, Sans. I love you. I love you. More than anything. More than anyone. Always. Forever.”

Sans blinked and tilted his head to the side, feeling a skeleton kiss brush his neck; a slightly unhinged smile gracing his features. Papyrus was here. His brother wouldn’t leave him alone again.

“i love you so damn much, paps.”

And in this silent Underground, they would be here together.

Always.


End file.
